You can use any wire, of course utilizing your knowledge of current requirements, voltage drop, etc. to AWG size. Also, keeping in mind voltage limits of the wire insulation. You could just mark the insulation for an inch or so on one end of the wire with a sharpie for plus or minus or both.
However, the multi-stranded silicone insulated wire is much more flexible and easier to use for all lead usage, in my experience of 40 years. Also, I prefer to use 16 AWG for all leads: large enough ampacity for most situations, yet small enough for flexibility. I use red for positive and black for negative, as well as a center tapped common. But that is just me.